The Claim: Exercising at Night Can Disrupt Your Sleep

THE FACTS -- Americans have long been encouraged to hit the gym or squeeze in a run around the block whenever possible, even at the end of a long workday. But can a late-evening workout ever be too late?

As a general rule, most fitness and sleep experts recommend avoiding intense physical activity in the immediate hours before bedtime, arguing that it takes at least three hours for adrenaline and other hormones that typically surge during a workout to return to normal levels.

But most studies have not found that to be the case. One study published in the journal Physiology and Behavior in 1998, for example, had a group of college students exercise moderately for about an hour on two separate nights, in one case 90 minutes before bedtime and in the other 30 minutes before bedtime.

The activity, the researchers found, had no significant effects on the amount of time the subjects needed to fall asleep. Nor did it affect any other factors indicating how well they slept, including duration of sleep and their number of "waking episodes" during the night. Other studies have had similar findings.

One researcher who has published widely on the subject, Dr. Shawn D. Youngstedt at the University of South Carolina, says he believes that exercise before bed can actually promote sleep, easing anxiety and raising body temperature. But the effects vary from person to person, he said.

THE BOTTOM LINE -- While experts generally recommend not exercising immediately before bedtime, most studies have found that it has no negative effects on sleep. ANAHAD O'CONNOR